Improve semchecking for duplicate cases in case statements (#7176)

* Improve semchecking for duplicate cases in case statements

* Revert to previous solution

* Improve test

compiler/nimsets.nim

@@ -151,6 +151,11 @@ proc complement*(a: PNode): PNode =
   for i in countup(0, high(x)): x[i] = not x[i]
   result = toTreeSet(x, a.typ, a.info)
 
+proc deduplicate*(a: PNode): PNode =
+  var x: TBitSet
+  toBitSet(a, x)
+  result = toTreeSet(x, a.typ, a.info)
+
 proc cardSet(s: PNode): BiggestInt =
   # here we can do better than converting it into a compact set
   # we just count the elements directly

compiler/semtypes.nim

@@ -497,8 +497,8 @@ proc semCaseBranchSetElem(c: PContext, t, b: PNode,
 
 proc semCaseBranch(c: PContext, t, branch: PNode, branchIndex: int,
                    covered: var BiggestInt) =
-
-  for i in countup(0, sonsLen(branch) - 2):
+  let lastIndex = sonsLen(branch) - 2
+  for i in 0..lastIndex:
     var b = branch.sons[i]
     if b.kind == nkRange:
       branch.sons[i] = b
@@ -516,14 +516,21 @@ proc semCaseBranch(c: PContext, t, branch: PNode, branchIndex: int,
         branch.sons[i] = skipConv(fitNode(c, t.sons[0].typ, r, r.info))
         inc(covered)
       else:
+        if r.kind == nkCurly:
+          r = r.deduplicate
+
         # first element is special and will overwrite: branch.sons[i]:
         branch.sons[i] = semCaseBranchSetElem(c, t, r[0], covered)
+
         # other elements have to be added to ``branch``
         for j in 1 ..< r.len:
           branch.add(semCaseBranchSetElem(c, t, r[j], covered))
           # caution! last son of branch must be the actions to execute:
-          var L = branch.len
-          swap(branch.sons[L-2], branch.sons[L-1])
+          swap(branch.sons[^2], branch.sons[^1])
+    checkForOverlap(c, t, i, branchIndex)
+
+  # Elements added above needs to be checked for overlaps.
+  for i in lastIndex.succ..(sonsLen(branch) - 2):
     checkForOverlap(c, t, i, branchIndex)
 
 proc semRecordNodeAux(c: PContext, n: PNode, check: var IntSet, pos: var int,

tests/casestmt/tduplicates.nim

@@ -0,0 +1,50 @@
+discard """
+  output: '''
+OK
+OK
+OK
+  '''
+"""
+
+type Kind = enum A, B
+var k = A
+
+template reject(b) =
+  static: doAssert(not compiles(b))
+
+reject:
+    var i = 2
+    case i
+    of [1, 1]: discard
+    else: discard
+
+reject:
+    var i = 2
+    case i
+    of 1, { 1..2 }: discard
+    else: discard
+
+reject:
+    var i = 2
+    case i
+    of { 1, 1 }: discard
+    of { 1, 1 }: discard
+    else: discard
+
+reject:
+    case k
+    of [A, A]: discard
+
+var i = 2
+case i
+of { 1, 1 }: discard
+of { 2, 2 }: echo "OK"
+else: discard
+
+case i
+of { 10..30, 15..25, 5..15, 25..35 }: discard
+else: echo "OK"
+
+case k
+of {A, A..A}: echo "OK"
+of B: discard